Components of net periodic benefit cost (Details 3) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension Benefits
Components of net periodic benefit cost (credit):
Service cost	$ 129	$ 155	$ 1,078
Interest cost	17,682	16,249	17,598
Expected return on assets	(21,218)	(19,917)	(23,536)
Amortization of prior service cost (credit)	71	71	(46)
Recognized net actuarial loss	4,869	7,173	7,070
Curtailment gain	0	0	(1,023)
Amortization of net transition obligation	0	0	0
Net periodic benefit cost (credit), including amount capitalized	1,533	3,731	1,141
Less amount capitalized	388	727	937
Net periodic benefit cost (credit)	1,145	3,004	204
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive (income) loss:
Net (gain) loss	77,238	(60,173)	19,982
Prior service cost (credit)	0	0	0
Amortization of actuarial loss	(4,869)	(7,173)	(7,070)
Amortization of prior service (cost) credit	(71)	(71)	1,069
Amortization of net transition obligation	0	0	0
Total recognized in accumulated other comprehensive (income) loss	72,298	(67,417)	13,981
Total recognized in net periodic benefit cost (credit) and accumulated other comprehensive (income) loss	73,443	(64,413)	14,185
Other Postretirement Benefits
Components of net periodic benefit cost (credit):
Service cost	1,518	1,675	1,747
Interest cost	3,521	3,215	4,166
Expected return on assets	(4,617)	(4,343)	(4,890)
Amortization of prior service cost (credit)	(1,393)	(1,457)	(1,438)
Recognized net actuarial loss	649	1,814	2,134
Curtailment gain	0	0	0
Amortization of net transition obligation	0	0	2,128
Net periodic benefit cost (credit), including amount capitalized	(322)	904	3,847
Less amount capitalized	(21)	164	910
Net periodic benefit cost (credit)	(301)	740	2,937
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive (income) loss:
Net (gain) loss	15,114	(30,461)	1,863
Prior service cost (credit)	0	0	(11,418)
Amortization of actuarial loss	(649)	(1,814)	(2,134)
Amortization of prior service (cost) credit	1,393	1,457	1,438
Amortization of net transition obligation	0	0	(2,128)
Total recognized in accumulated other comprehensive (income) loss	15,858	(30,818)	(12,379)
Total recognized in net periodic benefit cost (credit) and accumulated other comprehensive (income) loss	$ 15,557	$ (30,078)	$ (9,442)